Wireless communication technologies have rapidly developed over the last several years to the point where a mobile device, such as a mobile cellular telephone or a multi-function mobile wireless device (capable of email, internet browsing, voice calls, etc.), has the capability to operate in multiple wireless networks. For example, such a multi-mode device may be capable of operating in a wireless local area network (WLAN), such as a WiFi™ network and also with a wide area wireless network, such as a cellular network. For simplicity, a multi-mode or dual-mode wireless client device is referred to herein as a mobile node (MN). The MN may employ voice-over-internet protocol (VoIP) techniques when conducting a voice call via the WiFi network and more traditional circuit switching techniques when conducting a voice call via the cellular network. Consequently, the MN needs to reliably roam between the two different networks. This is particularly important when a user is in the middle of a voice call.
There are other applications where it may be desirable to handover VoIP service on a WiFi network to VoIP service on the cellular network. Further still, there are applications where it may be desirable to handover a data (non-voice call) communication session for a data application where continuity is beneficial. In all of these cases, a handover trigger generation by the WLAN infrastructure is useful.
WLAN infrastructure provides triggers to the MN or to a network component informing it when, upon entering a building, robust wireless VoIP service is available and when upon leaving the building, robust wireless VoIP service can no longer be provided. Handover triggers are provided by network infrastructure; user-based policies, network-based policies and network conditions (both wired and wireless) can be applied to determine if and when to send handover triggers.
In order to determine when to send handover triggers, the WLAN infrastructure measures link parameters on the radio link between a WLAN access point (AP) and the MN or obtains these measurements from the MN. In WiFi networks, the MN determines the AP with which to associate. As the MN roams from AP to AP, the link conditions naturally change. When the appropriate link parameters cross a threshold, the WLAN infrastructure generates and sends a handover trigger. In general, a handover trigger can be a hand-out trigger advising the MN to roam to a cellular network or a hand-in trigger advising the MN that it may roam in to the WiFi network.
One of the requirements for a WLAN handover system is to provide handover triggers without adversely affecting the battery life of the MN. One criterion users place on mobile devices is standby time and talk time. If the WLAN handover system is such that the MN must transmit many extra frames or keep its receiver on for extended periods of time, then the MN's battery will discharge too rapidly and the user will have to frequently recharge the MN and risk losing service during a critical user activity, e.g., an important voice call.
Techniques are needed to provide better handover service to MNs.